The present invention relates to a stepping motor driving apparatus which is suitably applicable to, for example, a liquid infusion pump used in the medical field.
Generally, a liquid infusion pump is used for feeding liquid under pressure from an upstream supply bag through a delivery tube to a downstream infusion cannula. The pump is driven by a stepping motor and a series of eccentric cams are mounted on a rotational shaft of the stepping motor. A finger is provided around a corresponding one of the eccentric cams so as to depress the delivery tube such that the liquid is fed under pressure.
FIG. 1 shows an arrangement of such a known liquid infusion pump. The known liquid infusion pump includes a pump head 1, a four-phase stepping motor 2 for driving the pump head 1 and a control unit 3 for outputting drive pulses to the stepping motor 2 and for controlling various portions of the known liquid infusion pump. Although not specifically shown, the pump head 1 depresses a delivery tube provided between a supply bag and an infusion cannula so as to feed liquid under pressure. Meanwhile, the control unit 3 is formed of a microcomputer.
The known liquid infusion pump further includes a key input portion 4 operable for adjusting the quantity of the liquid fed under pressure, etc., a display unit 5 for displaying pressure of the liquid, etc., an alarm indicator 6 for giving an alarm when pressure of the liquid is abnormal, etc. and a power source 7 for supplying electric power to the various portions of the known liquid infusion pump.
FIGS. 2(a) to 2(d) show wave forms of drive pulses supplied from the control unit 3 to excitation coils of the phases of the stepping motor 2, respectively. The stepping motor 2 is rotated through sequential excitation of the excitation coils.
In the known liquid infusion pump, a necessary quantity of the liquid is fed under pressure by maintaining the number of revolutions of the stepping motor 2 at a predetermined value. However, for example, in the case where torque of the stepping motor 2 is more than adequate for a given load owing to such factors as hardness of the delivery tube, drive current more than necessary is caused to flow through the stepping motor 2, thereby resulting in large power consumption.